This is easier than constructing a large circle of paper. Take two squares of paper. The width is fine if it approximates half that of the table. Fold both squares down middle forming rectangular halves. Unfold one square. Take one side that is divided by the fold and place that side's corners at the very edge of the table. Do the same with the other square, placing it so the fold lines of the two sheets of paper cross. The point where they cross is the center of the table.